Method of producing a silicon nitride sintered body

ABSTRACT

A silicon nitride sintered body having a high strength at high temperatures as well as at room temperature can be provided by the method of the present invention, which includes preparing a raw material consisting of SI 3  N 4  powder, a rare earth element oxide powder, and SiC powder, and at least one of a W compound powder and a Mo compound powder, forming the raw material into a shaped body, and then firing the shaped body in N 2  atmosphere to substantially crystallize the grain boundary phase of the Si 3  N 4  grains. The silicon nitride sintered body includes Si 3  N 4  as a main component, and the remainder of a rare earth element compound, SiC and at least one of a W compound and a Mo compound, the grain boundary phase of Sio 3  N 4  grains consisting substantially of crystal phases. The silicon nitride sintered body is dense and thin in color so that uneven coloring thereof can be decreased.

This is a division of application Ser. No. 07/649,770, filed Jan. 25,1991, now U.S. Pat. No. 5,096,859.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to silicon nitride sintered bodies havinghigh strength at high temperatures, and a method of producing the same.

2. Related Art Statement

Heretofore, as a silicon nitride sintered body containing an oxide of aGroup IIIa element including rare earth elements, a method of producinga sintered body has been disclosed wherein 85 mole % or more of Si₃ N₄is mixed with 15 mole % or less of at least one oxide of Group IIIaseries elements, shaped, and sintered in a non-oxidizing atmosphere, asdescribed in Japanese Patent publication No. 48-7,486. A and a siliconnitride sintered body has been disclosed which consists of at least 50wt. % of Si₃ N₄, not over than 50 wt. % of Y₂ O₃ or at least one oxideof La series elements, and 0.01-20 wt. % of Al₂ O₃, as described inJapanese Patent Publication No. 49-21,091.

However, there are problems in that the mere addition of a rare earthelement to silicon nitride can not produce a silicon nitride sinteredbody having high strength at high temperatures, and that the addition ofAl₂ O₃ results in low softening point and hence remarkably decreasedhigh temperature strength of the crystal grains boundary phase, thoughthe addition of Al₂ O₃ improves densification of the silicon nitridesintered body.

In order to solve the problem of high temperature strength, theapplicant disclosed a technique in Japanese Patent Application Laid-openNo. 63-100,067 wherein a desired composition and a desired quantityratio of rare earth element is added to Si₃ N₄ powder to specify thecrystal phase of the sintered body so as to achieve high strength athigh temperatures.

However, the silicon nitride sintered body disclosed in the JapanesePatent Application Laid-open No. 63-100,067 has a problem in that thoughit achieves high strength to certain extent at high temperatures thestrength is inferior to room temperature strength thereof. This isconsidered due to a small amount of glass phase remaining in thecomposition even after the crystallization of the grain boundary phase.In order to decrease the remaining glass phase, a method may beconsidered of calculating the entire amount of oxygen contained in theraw materials of silicon nitride into SiO₂ amount by conversion, andenlarging the quantity ratio of rare earth element oxide to SiO₂ in theraw materials so as not to leave glass phase in the sintered body as faras possible. However, the method has a problem in that the densifiedsintered body is difficult to produce.

SUMMARY OF THE INVENTION

An object of the present invention is to obviate the above problems andto provide a silicon nitride sintered body having high strength attemperatures from room temperature to high temperatures as well as amethod of producing the same.

The present invention is a silicon nitride sintered body consistingessentially of Si₃ N₄ as a main component, and the remainder of a rareearth element compound, SiC and at least one of a W compound and a Mocompound, the grain boundary phase of Si₃ N₄ grains consistingsubstantially of crystal phases.

The present invention is also a method of producing a silicon nitridesintered body, comprising, preparing a raw material consisting of Si₃ N₄powder, a rare earth element oxide powder, SiC powder, and at least oneof a W compound powder and a Mo compound powder, forming the rawmaterial into a shaped body, and then firing the shaped body in N₂atmosphere to substantially crystallize the grain boundary phase of theSi₃ N₄ grains.

The inventors have found out that in the above arrangement a siliconnitride sintered body having a Si₃ N₄ crystal grain boundary phaseconsisting substantially of a crystal phase can be obtained by addingSiC, and a W compound such as WC, and/or a Mo compound such as Mo₂ C toa Si₃ N₄ powder containing a desired rare earth element compound, suchas rare earth element oxide, mixing, and firing the mixture in N₂atmosphere. The silicon nitride sintered body can achieve high strengthat temperatures from room temperature to high temperatures.

Namely, the addition of a W compound and/or a Mo compound functions withthe rare earth element compound to accelerate the densification of thesintered body so as to mainly improve the strength at room temperature,while the addition of SiC is effective to accelerate the densificationof the sintered body as well as the crystallization of the grainboundary phase so as to mainly improve the strength at hightemperatures. The addition of SiC, and at least one of a W compound anda Mo compound functions synergistically to obtain a silicon nitridesintered body having high strength at temperatures from room temperatureto high temperatures.

Also, the addition of a W compound and/or a Mo compound has an effect ofdecreasing the frequent peculiar coloring of the Si₃ N₄ sintered bodydue to the addition of a rare earth element. Moreover, though thecoloring of the sintered body due to a rare earth element is notpreferable because the coloring changes when exposed to an oxygenicatmosphere, the color changing can be neglected owing to the decrease ofthe coloring due to the above effect. Furthermore, the addition of a Wcompound and/or a Mo compound functions synergistically with theaddition of SiC to decrease uneven coloring of the sintered body evenwhen the sintered body has a thick wall thickness.

BRIEF DESCRIPTION OF THE DRAWING

For a better understanding of the present invention, reference is madeto the accompanying drawing, in which:

FIG. 1 is a characteristic graph of an X-ray diffraction pattern of anexample of the silicon nitride sintered body of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The amount of addition of the rare earth element oxide is preferably2.7-10 mol %. If it is less than 2.7 mole %, a sufficient liquid phasefor the densification of the sintered body can not be obtained, while ifit exceeds 10 mole %, the densification can hardly be obtained even whenSiC, and at least one of a W compound and a Mo compound are added to theSi₃ N₄ powder containing a desired rare earth element compound. Morepreferably, it is 4.7-9 mole %. Generally, the optimum amount of therare earth element oxide varies depending on the silicon nitride rawmaterial used and is in a range of 2.7-10 mole %. Rare earth elementoxide other than Y₂ O₃ and Yb₂ O₃, such as Lu₂ O₃, Tm₂ O₃ or Er₂ O₃ maybe used. Mole % used herein is calculated by rare earth element oxidemole/{rare earth element oxide mole+Si₃ N₄ mole}.

As regards the raw material Si₃ N₄, those having large α content ispreferable from a viewpoint of sintering property. Desirably, oxygencontent thereof is 1-3 wt. %.

The amount of addition of SiC is desirably in a range of 0.5-11 wt. %relative to the reciped amount of silicon nitride and rare earth elementoxide. If it is less than 0.5 wt. %, a sufficient densification effectand crystallization acceleration effect can not be obtained. If itexceeds 11 wt. %, SiC prevents the densification of the sinteredproduct. More preferable amount is 1-5 wt. %. Any SiC of α type, β typeor amorphous type may be used.

The amount of addition of a W compound and/or a Mo compound is desirablyin a range of 0.5-3 wt. % relative to the reciped amount of siliconnitride and the rare earth element oxide. If it is less than 0.5 wt. %,a sufficient effect of improving the strength at room temperature cannot be exhibited. A more preferable amount is 1-2 wt. %.

In the method of the present invention, at first a mixture of the rawmaterial silicon nitride powder, SiC, and at least one of a W compoundand a Mo compound is prepared. Then, the resultant mixture is formed toa desired shape to obtain a shaped body. The obtained shaped body isfired at 1,700°-2,100° C., preferably at 1,900°-2,000° C. in N₂atmosphere under room pressure or under pressure, and crystallized by atemperature-decreasing process or a reheating treatment process.

Impurities, particularly cationic elements, such as Al, Fe, Mg, Ca, Naor K, in the raw materials to be used, desirably are not present in anamount of more than 0.5 wt. %, more preferably not more than 0.1 wt. %.Particle diameter of the raw materials is preferably as small aspossible from a viewpoint of sintering property and desirably not morethan 2 μm, more preferably not more than 1 μm.

Regarding the crystallization of the grain boundary phase, SiC has aneffect of accelerating the crystallization of the grain boundary phase,so that the temperature-decreasing process can sufficiently crystallizethe grain boundary phase, if the temperature-decreasing rate down to1,000° C. is not more than 100° C./min. If the crystallization of thegrain boundary phase is not sufficient due to a fastertemperature-decreasing rate than the above or due to some other cause, areheating treatment process may be effected to perform crystallization.Also, the reheating treatment process may be effected for a purpose ofremoving a residual stress in the sintered body, or the like purpose.The reheating treatment process is preferably effected at 1,300°-1,500°C. As a W compound and/or a Mo compound, WC or Mo₂ C is mentioned, butmetallic W or Mo, silicides or oxides of W or Mo may also be used.

If the raw materials are heat treated at 1,000°-1,500° C. in anoxidizing atmosphere so as to form a surface layer substantiallyconsisting of SiO₂ and a compound having a Re-Si-O structure (wherein Rerepresents a rare earth element) on the sintered body, the sintered bodycan exhibit a further high strength.

Hereinafter, the present invention will be explained in more detail withreference to examples.

EXAMPLE 1

The following materials are mixed in the ratios shown in Table 1 andground in a wet mill: a raw material powder of silicon nitride having apurity of 97 wt. %, an impurities cationic elements (Al, Fe, Mg, Ca, Naand K) content of a sum of not more than 0.1 wt. %, an oxygen content of2.2 wt. %, an average particle diameter of 0.6 μm, a BET specificsurface area of 17 m² /g, and an α content of 0.95; a rare earth elementoxide having a purity of 99 wt. %, an average particle diameter of0.3-2.0 μm, and characteristic properties as shown in Table 1; SiChaving a purity of 99 wt. %, an average particle diameter of 0.4 μm, aBET specific surface area of 20 m² /g; and either one of a W compoundhaving a purity of 99 wt. %, an average particle diameter of 0.4-3 μm,and a BET specific surface area of 0.5-10 m² /g and Mo₂ C having apurity of 99 wt. %, a particle diameter of 0.5-4 μm, and a BET specificsurface area of 0.3-10 m² /g. Then, water is removed from the groundmixture by evaporation, and the mixture is granulated to particles of adiameter of 150 μm to obtain a shaping powder. Thereafter, the shapingpowder is formed into shaped bodies of 50×40×6 mm, and fired at firingconditions as shown in Table 1 to obtain silicon nitride sintered bodiesof Nos. 1-20 of the present invention. SiC used in the sintered body isα type for No. 7, amorphous for No. 11, and β type for the othersintered bodies. Also, the same raw materials are used and reciped inthe reciping ratios as shown in Table 1, ground, granulated and formedin the same manner as described above, and fired at the firingconditions as shown in Table 1 to obtain silicon nitride sintered bodiesof Nos. 21-32 of comparative examples. Temperature-decreasing rate atthe firing is fundamentally 100° C./min down to 1,000° C., except forNo. 9 which used a temperature-decreasing rate of 110° C./min from aheat treatment temperature of 1,400° C. for 6 hrs in nitrogen.

These sintered bodies are measured on relative density, crystal phasesof grain boundary phase, and four-point bending strength at roomtemperature and 1,400° C. in the manners as described below, and theresults are shown in Table 1.

Relative density of the sintered bodies is determined by measuring abulk density of the sintered body based on the Archimedean principle,and calculating the ratio of the bulk density to the theoretical densityof the sintered body. The theoretical density is calculated from thecomposition of the reciped powders and the densities of the recipedpowders. The densities of the reciped powders are Si₃ N₄ : 3.2 g/cm³, Y₂O₃ : 4.8 g/cm³, Yb₂ O₃ : 9.2 g/cm³, Tm₂ O₃ : 8.8 g/cm³, Lu₂ O₃ : 9.4g/cm³, Er₂ O₃ : 8.6 g/cm³, SiC: 3.2 g/cm³, WC: 15.7 g/cm³, Mo₂ C: 9.2g/cm³, W: 19.2 g/cm³, Mo: 10.2 g/cm³, WO₃ : 7.2 g/cm³, MoO₃ : 4.7 g/cm³,WSi₂ : 9.4 g/cm³, and MoSi₂ : 6.3 g/cm³.

Four-point bending strength is measured according to JIS R-1601 "Methodof Testing a Bending Strength of Fine Ceramics".

Crystal phases of grain boundary phase are determined by an X-raydiffraction analysis using CuK α-ray. In Table 1, J is a crystal ofcaspedian structure, H is a crystal of apatite structure, K is a crystalof wollastonite structure, L is Ln₂ SiO₃ (Ln: rare earth element), and Sis a crystal expressed by Ln₂ SiO₇.

A result of an X-ray diffraction analysis of the silicon nitridesintered body of No. 6 of the present invention is shown in FIG. 1,wherein "a" represents b-Si₃ N₄, "b" represents J phase which is acrystal of caspedian structure, "c" represents H phase which is acrystal of apatite structure, "d" represents WSi₂, and "e" representsβ-SiC. As a result of chemical analyses of the sintered bodies, thechemical analyses coincided with compositions of the sintered bodiescalculated from the reciping ratios of the raw materials.

                                      TABLE 1                                     __________________________________________________________________________                                W     Mo                                                  Rare earth element                                                                            SiC compound                                                                            compound                                                                            Temper-                                       oxide           (ratio,                                                                           (ratio,                                                                             (ratio,                                                                             ature                                      No.                                                                              (wt %)     (mol %)                                                                            wt %)                                                                             wt %) wt %) (°C.)                          __________________________________________________________________________    Example                                                                             1 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 2:4                                                  2.7  2   WC 1  --    1900                                        2 Yb.sub.2 O.sub.3 = 15                                                                    5.9  3   WC 1  --    1700                                        3 Y.sub.2 O.sub.3 = 10                                                                     6.4  1   WC 1  --    1950                                        4 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 2:9                                                  4.7  2   WC 1  --    1900                                        5 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 2.9                                                  4.7  2   --    Mo.sub.2 C 1                                                                        2100                                        6 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  5   WC 1  --    1900                                        7 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  2   WC 1  --    2000                                         8                                                                              Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.9:16                                               9.0  2   WC 1  --    1900                                        9 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 4.2:17                                               10   2   WC 0.5                                                                              Mo.sub.2 C 0.5                                                                      1900                                       10 Y.sub.2 O.sub.3 :Tm.sub.2 O.sub.3 = 2:9                                                  4.7  7   --    Mo.sub.2 C 1                                                                        1950                                       11 Yb.sub.2 O.sub.3 :Lu.sub.2 O.sub.3 = 7:7                                                 4.6  3   WC 1  --    1900                                       12 Y.sub.2 O.sub.3 :Er.sub.2 O.sub.3 = 2:3                                                  6.6  1   WC 1  --    1900                                       13 Y.sub.2 O.sub.3 = 10                                                                     6.4  1   W 1   --    1900                                       14 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  2   WO.sub.3 1                                                                          --    1900                                       15 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  1   WO.sub.3 3                                                                          --    1900                                       16 Y.sub.2 O.sub.3 :Er.sub.2 O.sub.3 = 2:3                                                  6.6  1   WSi.sub.2 1                                                                         --    1950                                       17 Yb.sub.2 O.sub. 3 = 15                                                                   5.9  3   --    Mo 2  1850                                       18 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  2   --    MoO.sub.3 1                                                                         1900                                       19 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  2   --    MoO.sub.3 3                                                                         1900                                       20 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 4.2:17                                               10   2   --    MoSi.sub.2 1                                                                        1950                                  Compar-                                                                            21 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 2:4                                                  2.7  2   --    --    1900                                  ative                                                                              22 Yb.sub.2 O.sub.3 = 15                                                                    5.9  --  --    Mo.sub.2 C 1                                                                        1700                                  Example                                                                            23 Yb.sub.2 O.sub.3 = 10                                                                    6.4  --  WC 1  --    1950                                       24 Y.sub.2 O.sub.3 :Tm.sub.2 O.sub.3 = 2:9                                                  4.7  7   --    --    1950                                       25 Yb.sub.2 O.sub.3 :Lu.sub.2 O.sub.3 = 7:7                                                 4.6  3   --    --    1900                                       26 Y.sub.2 O.sub.3 :Er.sub.2 O.sub.3 = 2:3                                                  6.6  1   --    --    1900                                       27 Y.sub.2 O.sub.3 = 10                                                                     6.4  --  W 1   --    1900                                       28 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  --  WO.sub.3 1                                                                          --    1900                                       29 Y.sub.2 O.sub.3 :Er.sub.2 O.sub.3 = 2:3                                                  6.6  --  WSi.sub.2 1                                                                         --    1950                                       30 Yb.sub.2 O.sub.3 = 15                                                                    5.9  --  --    Mo 2  1850                                       31 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  --  --    MoO.sub.3 1                                                                         1900                                       32 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 4.2:17                                               10   --  --    MoSi.sub.2 1                                                                        1900                                  __________________________________________________________________________                              Bending                                                              Pres-                                                                             Relative                                                                           strength MPa                                                      Time                                                                             sure                                                                              density                                                                            room     Crystal phases                                        No.                                                                              (hr)                                                                             (atm)                                                                             (%)  temp.                                                                             1400° C.                                                                    other than Si.sub.3 N.sub.4                __________________________________________________________________________    Example     1 2  10  99   960 840  H, L, S, WSi.sub.2, SiC                                2 3   1  99   960 840  J, WSi.sub.2, SiC                                      3 2  50  99   1000                                                                              850  J, H, WSi.sub.2, SiC                                   4 2  10  99   980 850  H, J, WSi.sub.2, SiC                                   5 2  100 99   970 850  J, H, K, MoSi.sub.2, SiC                               6 2  10  99   990 860  J, H, WSi.sub.2, SiC                                   7 2  100 99   990 870  J, WSi.sub.2, SiC                                      8 2  10  99   980 860  J, WSi.sub.2, SiC                                      9 2  10  99   950 840  J, WSi.sub.2, MoSi.sub.2, SiC                         10 2  20  99   950 860  H, L, MoSi.sub.2, SiC                                 11 2  10  99   970 860  J, WSi.sub.2, SiC                                     12 2  10  99   980 860  J, WSi.sub.2, SiC                                     13 2  10  99   950 840  J, H, WSi.sub.2, SiC                                  14 2  10  99   990 870  J, H, WSi.sub.2, SiC                                  15 2  10  99   970 840  J, H, WSi.sub.2, SiC                                  16 2  50  99   980 860  J, WSi.sub. 2, SiC                                    17 3  10  99   950 840  J, MoSi.sub.2, SiC                                    18 2  10  99   960 860  J, H, MoSi.sub.2, SiC                                 19 2  10  99   970 860  J, H, MoSi.sub.2, SiC                                 20 1  50  99   940 840  J, H, MoSi.sub.2, SiC                      Compar-    21 2  10  98   680 680  L, S, SiC                                  ative      22 3   1  97   780 770  J, SiC                                     Example    23 2  50  98   850 700  J, H, WSi.sub.2                                       24 2  20  98   760 690  H, L                                                  25 2  10  98   780 780  J                                                     26 2  10  99   800 800  J                                                     27 2  10  99   810 700  J, H, WSi.sub.2                                       28 2  10  99   840 710  J, H, WSi.sub.2                                       29 2  50  99   830 690  J, H, WSi.sub.2                                       30 3  10  99   810 710  J, MoSi.sub.2                                         31 2  10  99   810 700  J, H, MoSi.sub.2                                      32 1  10  99   820 680  J, H, MoSi.sub.2                           __________________________________________________________________________     J: cuspidine structure,                                                       H: apatite structure,                                                         K: wollastonite structure,                                                    L: Re.sub.2 SiO.sub.5 (Re: rare earth element)                                 S: Re.sub.2 Si.sub.2 O.sub.7 (Re: rare earth element)                   

As clearly apparent from the above Table 1, the sintered bodies Nos.1-20 containing a rare earth element oxide, SiC and at least one of a Wcompound and a Mo compound have high strength both at room temperatureand 1,400° C. In contrast, the sintered bodies of Nos. 21 and 24-26containing merely a rare earth element and SiC and the sintered bodiesof Nos. 22, 23 and 27-32 containing merely a rare earth element andeither one of a W compound and a Mo compound, have low strength.

EXAMPLE 2

The same raw materials as those of Example 1 are used and reciped inreciping ratios as described in Table 2, mixed and ground in a wet typemill. Thereafter, water is removed from the ground mixture byevaporation, and the mixture is granulated to particles of a diameter of150 μm to obtain a shaping powder. Then, the shaping powder is formedinto shaped bodies of a size of 50×40×6 mm, and fired at firingconditions as described in Table 2 to obtain the silicon nitridesintered bodies of Nos. 33-51 of the present invention. The siliconnitride sintered bodies of Nos. 48-51 are respectively heat treated inan oxidizing atmosphere at 1,000° C.×5 hrs, 1,200° C.×2 hrs, 1,300° C.×1hr, or 1,500° C.×1 hr. Also, the same raw materials are used and recipedin the reciping ratios as shown in Table 2, ground, granulated andformed in the same manner as described above, and fired at the firingconditions as described in Table 2 to obtain silicon nitride sinteredbodies of Nos. 52-55 of comparative examples. SiC used is β type.Temperature-decreasing rate from the firing temperature down to 1,000°C. is 100° C./min.

These sintered bodies are measured on bulk density, crystal phases andfour-point bending strength at room temperature and 1,400° C. Theresults are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                                WC    Mo.sub.2 C                                          Rare earth element                                                                            SiC compound                                                                            compound                                                                            Temper-                                       oxide           (ratio,                                                                           (ratio,                                                                             (ratio,                                                                             ature                                      No.                                                                              (wt %)     (mol %)                                                                            wt %)                                                                             wt %) wt %) (°C.)                          __________________________________________________________________________    Example                                                                            33 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  0.5 1     --    1900                                       34 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  1   1     --    1900                                       35 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  2   1     --    1900                                       36 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  3   1     --    1900                                       37 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  5   1     --    1900                                       38 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  7   1     --    1900                                       39 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  11  1     --    1900                                       40 Y.sub.2 O.sub.3 :Yb.sub. 2 O.sub.3 = 3.4:14                                              7.8  11  1     --    1900                                       41 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  2   0.5   --    1900                                       42 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  2   1.5   --    1900                                       43 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  2   2     --    1900                                       44 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  2   3     --    1900                                       45 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  1   --    1     1900                                       46 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  3   --    1     1900                                       47 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  2   --      1.5 1900                                       48 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  2   1     --    1900                                       49 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  2   1     --    1900                                       50 Y.sub.2 O.sub. 3 :Yb.sub.2 O.sub.3 = 3.4:14                                              7.8  2   1     --    1900                                       51 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  2   --    1     1900                                  Compar-                                                                            52 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  1   --    --    1900                                  ative                                                                              53 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  --  1     --    1900                                  Example                                                                            54 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  --  --    1     1900                                       55 Y.sub.2 O.sub.3 :Yb.sub.2 O.sub.3 = 3.4:14                                               7.8  --  0.5     0.5 1900                                  __________________________________________________________________________                         Bending                                                                   Rela-                                                                             strength                                                              Pres-                                                                             tive                                                                              MPa      Crystal  Surface                                          Time                                                                             sure                                                                              density                                                                           room     phases other                                                                           crystal                                       No.                                                                              (hr)                                                                             (atm)                                                                             (%) temp.                                                                             1400° C.                                                                    than Si.sub.3 N.sub.4                                                                  phase                                  __________________________________________________________________________    Example                                                                              33 2  10  99  940 840  J, H, WSi.sub.2, SiC                                                                   --                                            34 2  10  99  970 860  J, H, WSi.sub.2, SiC                                                                   --                                            35 2  10  99  1000                                                                              870  J, H, WSi.sub.2, SiC                                                                   --                                            36 2  10  99  1000                                                                              870  J, H, WSi.sub.2, SiC                                                                   --                                            37 2  10  99  990 860  J, H, WSi.sub.2, SiC                                                                   --                                            38 2  10  99  940 840  J, H, WSi.sub.2, SiC                                                                   --                                            39 2  10  99  930 830  J, H, WSi.sub.2, SiC                                                                   --                                            40 4  10  99  940 840  J, H, WSi.sub.2, SiC                                                                   --                                            41 2  10  99  930 840  J, H, WSi.sub.2, SiC                                                                   --                                            42 2  10  99  990 870  J, H, WSi.sub.2, SiC                                                                   --                                            43 2  10  99  990 860  J, H, WSi.sub.2, SiC                                                                   --                                            44 2  10  99  950 840  J, H, WSi.sub.2, SiC                                                                   --                                            45 2  10  99  950 860  J, H, MoSi.sub.2, SiC                                                                  --                                            46 2  10  99  950 860  J, H, MoSi.sub.2, SiC                                                                  --                                            47 2  10  99  950 860  J, H, MoSi.sub.2 , SiC                                                                 --                                            48 2  10  99  1020                                                                              880  J, H, WSi.sub.2, SiC                                                                   S, L                                          49 2  10  99  1030                                                                              880  J, H, WSi.sub.2, SiC                                                                   S, L, SiO.sub.2                               50 2  10  99  1030                                                                              890  J, H, WSi.sub.2, SiC                                                                   S, L, SiO.sub.2                               51 2  10  99  1020                                                                              880  J, H, MoSi.sub.2, SiC                                                                  S, L, SiO.sub.2                        Compar-                                                                              52 2  10  99  810 810  J, H, SiC                                                                              --                                     ative  53 2  10  99  860 690  J, H, WSi.sub.2                                                                        --                                     Example                                                                              54 2  10  99  860 700  J, H, MoSi.sub.2                                                                       --                                            55 2  10  99  860 690  J, H, WSi.sub.2, MoSi.sub.2                                                            --                                     __________________________________________________________________________     J: cuspidine structure,                                                       H: apatite structure,                                                         K: wollastonite structure,                                                    L: Re.sub.2 SiO.sub.5 (Re: rare earth element)                                S: Re.sub.2 Si.sub.2 O.sub.7 (Re: rare earth element)                    

As clearly apparent from the above Table 2, the effect of addition of WCor Mo₂ C is respectively seen particularly in the increase of strengthof the sintered bodies at room temperature, viewed from a comparison ofExample No. 34 and Comparative Example No. 52 and a comparison ofExample No. 45 and Comparative Example No. 52.

As apparent from the foregoing explanations, according to the method ofproducing a silicon nitride sintered body of the present invention, SiCand at least one of a W compound and a Mo compound are added to Si₃ N₄powder containing a desired rare earth element oxide, and fired in N₂atmosphere, so that the grain boundary phase of Si₃ N₄ grains can besubstantially made of crystal phases and hence a silicon nitridesintered body can be obtained having a high strength at temperaturesfrom room temperature to high temperatures.

Although the present invention has been explained with reference tospecific examples and numerical values, it will be of course apparent tothose skilled in the art that various changes and modifications thereofare possible without departing from the broad aspect and scope of thepresent invention as defined in the appended claims.

What is claimed is:
 1. A method of producing a silicon nitride sinteredbody containing Si₃ N₄ grains comprising the steps of:preparing a rawmaterial consisting essentially ofa) Si₃ N₄ powder as a main component,b) 2.7-10 mol % of a rare earth element compound powder, calculated asan oxide of the rare earth element relative to a sum of mol % of Si₃ N₄and mol % of the rare earth element compound calculated as an oxide ofthe rare earth element, c) 0.5-11 wt. % SiC powder relative to a sum ofSi₃ N₄ and the rare earth element compound calculated as an oxide of therare earth element, and d) 0.5-3 wt. % of at least one of a W compoundpowder or metallic W and a Mo compound powder 14 or metallic Mo relativeto a sum of Si₃ N₄ and the rare earth element compound calculated as anoxide of the rare earth element; forming the raw material into a shapedbody; and firing the shaped body at 1,700°-2,100° C. in N₂ atmosphere tosubstantially crystallize a grain boundary phase of the Si₃ N₄ grains.2. The method of claim 1, wherein the rare earth element of said rareearth element compound is selected from the group consisting of Y andYb.
 3. The method of claim 1, wherein said W compound is at least onecompound selected from the group consisting of carbides of W, oxides ofW, and silicides of W, and said Mo compound is at least one compoundselected from the group consisting of carbides of Mo, oxides of Mo, andsilicides of Mo.
 4. The method of claim 1, wherein said shaped body isfired at 1,900°-2,000° C.